The Cryogenian is the second period of the Neoprotozoic era, from 850 million to 630 million years ago. It followed the Tonian and preceded the Edicarian. It is subdivided into 2 epochs: the Sturtian 850 to 650 million years ago and the Varangian 650 to 630 million years ago.
It is thought that this period on Earth suffered a gigantic glaciation extending at least to the tropics. This is the "snowball Earth" theory; The debate is continuing on this subject and sometimes the "slushball earth" theory is proposed in which the Earth was not completely covered in ice.